The following abbreviations are herewith defined:    3GPP third generation partnership project    ARQ automatic repeat request    AS access stratum    BCCH broadcast control channel    BLER block error rate    CQI channel quality information    C-RNTI C-plane radio network temporary identifier    DL downlink (network to UE)    eNB Node B/base station in an evolved UTRAN system    E-UTRAN evolved UMTS radio access network    GSM global system for mobile communications    HARQ hybrid ARQ    HO handover    LTE long term evolution    MME mobility management entity    NW network    PCH paging channel    PDSCCH physical downlink shared control channel    PDSCH physical downlink shared channel    PSTN public switched telephone network    RAT radio access technology    RLC radio link control    RRM radio resource management    RX receive    SAE system architecture evolution    SIB system information block    SNIR signal to noise interference ratio    TX transmit    UE user equipment    UL uplink (UE to network)    UMTS universal mobile telecommunications system    UPE user plane entity    UTRAN UMTS terrestrial radio access network    VoIP voice of internet protocol
Handover of a UE between base stations is well known in the art, and different wireless protocols handle specific implementations differently. Known are soft and hard handovers, as well as specific requirements for inter-RAT handovers and handovers between base stations under control of different radio network controllers. Advantages of the invention detailed below are pronounced within the E-UTRAN system and so that system will be discussed as context, though as will be seen the signaling detailed herein may be employed with any wireless protocol that hands over UEs between different network entities. E-UTRAN (commonly termed 3.9G or simply LTE) is a packet-data-based transmission system that supports intra-frequency, inter-frequency and intra-RAT handover. It has been agreed that LTE will use a break-before-make backward hard handover. Soft handover will not be used.
One challenging scenario for executing reliable handovers is termed an urban canyon, shown in FIG. 1A which is reproduced from FIG. 1 of 3GPP TSG-RAN WG-2 MEETING #56 (Qualcomm Europe, 6-10 Nov. 2006, Riga, Latvia). Base stations 1 through 4 control the areas demarcated by the hexagonal dotted lines in which they are respectively centered. The UE path is along the street between BS1 and BS2, and that path spans a cross street depicted. Assume buildings at the corners of the intersection interfere with line of sight signaling between the various base stations and the moving UE. As the UE is at or very near the intersection, its received signal strength from base stations 2 and 3 increases very rapidly, and drops off again as it passes through the intersection and the building again block signals from those laterally disposed base stations.
The problem is particularly detailed at FIG. 1B, reproduced from FIG. 4 of 3GPP TSG-RAN WG-2 MEETING #56BIS (Qualcomm Europe, 16-19 Jan. 2007, Sorrento, Italy). That same document illustrates other scenarios wherein high frequency reuse factors are employed (e.g., GSM and UMTS), but FIG. 1B relates to LTE as is currently developed and seen to be the most challenging urban canyon since LTE uses full or fractional reuse among the various base stations. As the UE approaches the intersection, it will start receiving rapidly increasing signal from eNBs “2” and “3”. As the UE enters the intersection, the signal from eNBs “2” or “3” becomes stronger than eNB “1”. Assuming that the link between eNB “1” and the UE remains reliable, regardless whether full or fractional frequency reuse is employed it is likely that the network grants handover to one of them, say eNode B “2”. As the UE leaves the intersection, the signal strength between eNB “2” and the UE rapidly drops and by the time UE is able to measure its signal strength applying proper filtering, the link with eNB “2”, and therefore the link between the network and the UE, is lost. Interruption of service to the UE in the urban canyon scenario as shown in FIG. 1B can be routinely expected when the serving cell is a function of UE location.
Typically, intra-frequency handover is triggered when the UE is at the cell edge of current serving cell. Softer HO (intra-eNB) is an implementation issue for the network alone, and typically will not rely on signaling with the UE other than what is normally required in an inter-eNB handover. In a frequency reuse-1 system such as LTE, the UE may experience strong interference and low SNIR (e.g. <−5 db) in the DL when at or near a cell edge. Given such low SNIR, the BLER of a message after ARQ and HARQ will be still very high in some scenarios such as the urban canyon detailed above. This raises the concern whether HO execution signaling (mainly the HO command in DL) can be reliably transmitted between the eNB and the UE. This is a coverage problem in LTE since unreliable HO signaling can result in the UE losing coverage completely and having to execute cell reselection procedures, which interrupts the user's service and increases control signaling required to re-establish the UE on a cell.
According to preliminary simulation results by the inventors, there is a high probability of a failed (not correctly received by the UE) HO command when the network load is high and/or UE mobility is high (e.g. >30 km/hour), and/or small receiver diversity (e.g. 1 receiver antenna). If the transmission of the HO command failed, the UE will move to the idle state and start cell reselection. The whole procedure due to the failure of the HO command will cause significant service interruption time, and occupy additional bandwidth for UE reestablishment. Frequent radio link failure is especially to be avoided in VoIP applications.
What is needed in the art then is a way to increase the reliability of HO signaling. The above documents from which FIGS. 1A-1B are taken propose a solution that is seen to require significant architectural changes. A more elegant solution is detailed below that is applicable without substantial changes to existing system architecture, whether that system is LTE or otherwise.